DayZ's standalone was slated for a 2012 release, but that date might not be set in stone anymore, as the Arma II mod undergoes an engine change.
Dean "Rocket" Hall's Arma II mod, DayZ, has been a work in progress for a while now, but it was meant to have a standalone release before the end of the year. This may not happen, explains the developer. Accoirding to Hall, though they are still aiming for a 2012 release, the developers of the mod would not be entirely opposed to pushing the release back, if it meant giving the game a more solid foundation. Hall has stated that short term goals such as meeting a deadline, should not compromise long term goals, such as the overall quality of the game.
One of the reasons there may be a delay, is the extensive overhaul of the game being implemented, most interesting of which is a new engine. Though the engine may not look drastically different from the previous engine, it implements a lot of functionality changes at it's core, making weapons customizable, as well as clothes and changes in controls and the map. The game, when it is released, will launch on Steam.